1. Field of the Invention
The invention relates to the field of MOS buffer circuits, particularly inverters which convert lower level signals to higher level signals.
2. Prior Art
In metal-oxide-semiconductor (MOS) integrated circuits there is a frequent requirement to convert lower level signals to higher level signals. For example, a TTL or ECL input such as a clocking signal for an MOS memory, must be converted to a higher level signal compatible with MOS circuits. Such conversion must be performed at high speeds and ideally at low power levels.
Often in MOS circuits higher speed circuits are realized by fabricating larger devices which consume more power, thus higher speed is obtained at the cost of increased power. However, in an input inverter circuit, it is not generally possible to increase the speed of a circuit beyond a predetermined limit even if more power is consumed. That is, the circuit is "gain/bandwidth" limited. Specifically, assume that a load device is coupled in series with an input MOS field-effect transistor, the gate of which receives a lower level input signal such as a TTL level signal. It would appear that the rise time of a signal sensed at the load device (output node) could be substantially increased by reducing the impedance of this load device. However, in order to reduce this impedance the input transistor, of necessity, must become larger. This adds capacitance to the output node, thereby slowing the rise time of the output signal. With current production MOS n-channel processing, rise times at such nodes of approximately 40 nanoseconds are obtainable. In such circuits the decay time is generally not the limiting factor in determining the circuit speed since the large input transistor quickly pulls-down the output node.
In the invented circuit, the capacitance associated with the large input transistor is decoupled from the output node or line as the potential on this node rises. This permits substantial improvements in performance; rise times of approximately 11 nanoseconds have been obtained with current production n-channel processing.